If {ARCHIVE,LIBRARY,RUNTIME}_OUTPUT_DIRECTORY is set with a genex then
do not add the per-config subdirectory on multi-config generators.
This will allow projects to use $<CONFIG> to place the per-config
part of the directory path somewhere other than the end.
Add sections to the cmake-buildsystem(7) manual and cross-reference
them with relevant variables and target properties. This avoids
duplicating the information and allows it to be more detailed.